Today I Learned

[DrGreg]

TIL that the largest known galaxy has a diameter of 1,764,000 light years.

To put that into perspective, that's about 20 times the diameter of our own galaxy (again according to Wikipedia).

(Caveat: all these numbers are estimates, and different estimation methods can give substantially different answers, yet again according to Wikipedia -- this is a subject I know nothing about.)

 

[BWV]

To put that into perspective, that's about 20 times the diameter of our own galaxy (again according to Wikipedia).

(Caveat: all these numbers are estimates, and different estimation methods can give substantially different answers, yet again according to Wikipedia -- this is a subject I know nothing about.)

Or over half the distance to Andromeda

 

[pinball1970]

Or over half the distance to Andromeda

68%!
First thing I did!

 

[DaveC426913]

Or over half the distance to Andromeda

To put that in perspective...

 

[Ivan Seeking]

TIL that the largest known galaxy has a diameter of 1,764,000 light years.

What is the centripetal force acting on an object on the outermost edge of that galaxy?

 

[DaveC426913]

What is the centripetal force acting on an object on the outermost edge of that galaxy?

I should clarify that - despite my deliberate artistic licensing - it's actually an elliptical galaxy.

The force shouldn't be too hard to approximate (by pretending it's of uniform density).
Mass: 2.3×10¹⁴M☉ (230 trillion Sols) = 4x10⁴⁴kg
Distance: 882kly = 8.3x10¹⁸km

(I'm doing something wrong; I get like 7x10²⁰Newtons). I'll show my work...

M = 2.3 x 10^{14}\\D=8.8x10^5 light years\\<br /> (1 light year = 9.4 x10{12} )

_{(OK, I give up on LaTeX or MathJax or whatever it is. The commands they say work don't work.)}

 

[strangerep]

I should clarify that - despite my deliberate artistic licensing - it's actually an elliptical galaxy.

The force shouldn't be too hard to approximate (by pretending it's of uniform density).
Mass: $2.3\times 10^{14}$ M☉ (230 trillion Sols) = $4 \times 10^{44}$ kg
Distance: 882kly = 8.3x10¹⁸km

(I'm doing something wrong; I get like 7x10²⁰Newtons). I'll show my work...

$$ M = 2.3 x 10{14} ~~;~~ D=8.8x10^5 \mbox{light years} $$
(1 light year = 9.4E12 ?what?

I don't know what you're trying to do. Were you trying to use $a = GM/r^2$ ?

Regardless, you'll have to use a MONDian modification of the Newtonian formula to get anywhere near the actual physical result.

(OK, I give up on LaTeX or MathJax or whatever it is. The commands they say work don't work.)

They do work if you RTFM. I fixed yours a bit.

 

[strangerep]

What is the centripetal force acting on an object on the outermost edge of that galaxy?

If it were a spiral, you should be able to get a reasonable approximation to the outer tangential velocity using the Tully-Fisher/MONDian formula $v_{\text{tan}}^4 = GM a_0$, where $a_0$ is the Milgrom constant. From that you can calculate centripetal acceleration.

Elliptical galaxies are a bit different, however. One must use the Faber-Jackson relation between velocity dispersion and luminosity, but I won't attempt to explain that here since it involves some tricks such as the Virial theorem.

 

[DrGreg]

What is the centripetal force acting on an object on the outermost edge of that galaxy?

To be pedantically and relativistically correct, the answer is zero.

(Gravity isn't a force in relativity, but of course in Newtonian theory it is a force.)

 

[Ivan Seeking]

To be pedantically and relativistically correct, the answer is zero.

(Gravity isn't a force in relativity, but of course in Newtonian theory it is a force.)

Heh. I doubt we get into velocities great enough or gravity strong enough to require GR. But if the word "force" doesn't apply at all, perhaps it should be dropped altogether. Why are we teaching false physics?

Teaching Newtonian physics makes sense because it is accurate enough for most applications. But calling something a force when it isn't actually a force is another matter.

 

[DaveC426913]

I don't know what you're trying to do. Were you trying to use $a = GM/r^2$ ?
They do work if you RTFM. I fixed yours a bit.

I was just trying to show my work - one equation per line. I RTFM'd and it said that "\\\" starts a new line but it does not.

 

[Ibix]

I was just trying to show my work - one equation per line. I RTFM'd and it said that "\\\" starts a new line but it does not.

You need the eqnarray* environment. It's basically a three column table with the columns right, center, and left aligned. Within that environment, & delimits the columns, \\\\ ends rows. Like this:$$\begin{eqnarray*}
F&=&ma\\
&=&\frac{GMm}{r^2}
\end{eqnarray*}$$Omit the asterisk in the environment name if you want your equations to be numbered.

 

[Ivan Seeking]

Easier

i

 

[pinball1970]

TIL Euclid has released first light commissioning images.

https://www.esa.int/ESA_Multimedia/Images/2023/07/Early_commissioning_test_image_VIS_instrument

 

[BillTre]

The Great Carrier Reef; probably mostly dead now due too high water temperatures off Florida.

 

[pinball1970]

TIL learned NASA found Voyager 2 again. TODIL (the other day I learned) That they had lost it but I thought I'd wait till they found it again before posting.

 

[DaveC426913]

The Great Carrier Reef; probably mostly dead now due too high water temperatures off Florida.View attachment 330001

I wonder if its harboring any Swordfish or Barracuda...

 

[berkeman]

TIL learned NASA found Voyager 2 again. TODIL (the other day I learned) That they had lost it but I thought I'd wait till they found it again before posting.

Oops...

CNN —

The Voyager mission team at NASA has been able to detect a signal from Voyager 2 after losing contact with the spacecraft, which has been operating for nearly 46 years.

“We enlisted the help of the (Deep Space Network) and Radio Science groups to help to see if we could hear a signal from Voyager 2,” said Suzanne Dodd, Voyager’s project manager at NASA’s Jet Propulsion Laboratory in Pasadena, California. “This was successful in that we see the ‘heartbeat’ signal from the spacecraft. So, we know the spacecraft is alive and operating. This buoyed our spirits.”

Commands sent to Voyager 2 on July 21 accidentally caused the spacecraft’s antenna to point 2 degrees away from Earth. The miniscule shift means that Voyager 2 can’t receive any commands from mission control or send data back to Earth from its location more than 12.3 billion miles (19.9 billion kilometers) in interstellar space.

The mission team was pleasantly surprised to be able to detect the spacecraft’s “carrier signal” using the Deep Space Network, an international array of massive radio antennas that allows NASA to communicate with missions across the cosmos.

Each of the three giant dishes are equidistant, meaning that one is always in communication with different spacecraft as Earth rotates. One radio antenna is located at Goldstone near Barstow, California, the second near Madrid, and the third near Canberra, Australia.

Now, the mission team will attempt to send a signal back to the spacecraft.

“We are now generating a new command to attempt to point the spacecraft antenna toward Earth,” Dodd said. “There is a low probability that this will work.”

https://www.cnn.com/2023/08/01/world/voyager-2-communication-blackout-scn/index.html

(emphasis mine) That's kind of depressing...

 

[pinball1970]

Oops...https://www.cnn.com/2023/08/01/world/voyager-2-communication-blackout-scn/index.html

(emphasis mine) That's kind of depressing...

Oh..that IS TIL and IS v depressing.

Some PhD student, "what's this dude?"
PhD "Dude! Don't touch that!"
Some PhD student, "Dude don't panic, just the the dish in Oz, see what happens if you change the orientation by a few %."
PhD "That's actual V2, you didn't touch it right?"
Some PhD student, "It's ok, just turned it back. No harm."
PhD "Ok. Except the first command will mean the second command will not be received."
Some PhD, "Dude, of course yeah, I'm learning so much here."

 

[strangerep]

[...]
Some PhD, "Dude, of course yeah, I'm learning so much here."

That reminds me of when I had the task of "supervising" a 3rd yr compsci student over the Dec-Jan univ holidays. He wondered what would happen if he adjusted an aircon dial in our main computer room. He caused an instant precautionary shutdown of our main server -- during a peak usage time. Yeah, he learned that lesson... (sigh)

 

[kyphysics]

 

[DaveC426913]

You know, if I had encountered this on a real test, it is quite possible I'd have gotten it wrong.
Not because I don't know my grammar but because I read through it too fast and interpolated words that weren't there.

 

[NTL2009]

You know, if I had encountered this on a real test, it is quite possible I'd have gotten it wrong.
Not because I don't know my grammar but because I read through it too fast and interpolated words that weren't there. ...

I did the same on first reading, mentally inserting "practicING" where it said "practice", because that is what our mind does when we read - like that famous example of a paragraph with every single word grossly misspelled, yet it is easily read, because the first and last letters are correct (and maybe the right number of letters? I forget). But since I didn't see any error in the first go-around, I took a closer look the second, and caught it, so I do think I would have got it on a test.

Nit -pick (for fun), since"practicING" is longer than "practice", I think you "extrapolated", rather than "interpolated"! :)

Ahhh, here it is - the trick is first/last letters correct, middle letters mixed up, but the right letters:

Aoccdrnig to a rscheearch at Cmabrigde Uinervtisy, it deosn’t mttaer in waht oredr the ltteers in a wrod are, the olny iprmoetnt tihng is taht the frist and lsat ltteer be at the rghit pclae. The rset can be a toatl mses and you can sitll raed it wouthit porbelm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe.

 

[gmax137]

Nit -pick (for fun), since"practicING" is longer than "practice", I think you "extrapolated", rather than "interpolated"! :)

or, interpolated the word "to"

They began practice their piano duet
becomes
They began to practice their piano duet

 

[DaveC426913]

I did the same on first reading, mentally inserting "practicING" where it said "practice", because that is what our mind does when we read

Me, I think I assumed the word 'to' before 'practice'.

 

[Jonathan Scott]

I did the same on first reading, mentally inserting "practicING" where it said "practice", because that is what our mind does when we read ...

In UK English, the word "practice" as a verb is already wrong in this context; it should be "practise" (the spelling follows the same scheme as advise/advice). So I considered that one wrong even before I spotted the missing "to"!

 

[gmax137]

TIL:

In UK English, the word "practice" as a verb ... should be "practise" (the spelling follows the same scheme as advise/advice).

I did not know that!

 

[pinball1970]

That reminds me of when I had the task of "supervising" a 3rd yr compsci student over the Dec-Jan univ holidays. He wondered what would happen if he adjusted an aircon dial in our main computer room. He caused an instant precautionary shutdown of our main server -- during a peak usage time. Yeah, he learned that lesson... (sigh)

My previous company was pretty good in terms of student placement and summer jobs. We got a lot of foreign students too.
Great fun trying to learn the languages.
From memory no real gaffs, they were smart kids.
Also from memory I got absolutely nowhere with Portuguese.

 

[DaveC426913]

(the spelling follows the same scheme as advise/advice).

Does the pronunciation follow the same scheme?

English people: "Just going into my dental office to practiss my practizz."

 

[Jonathan Scott]

Does the pronunciation follow the same scheme?

No, "practice" and "practise" in UK (and Australian) English usually sound the same. And even though the verb form is "practise", it is very common for people to get it wrong, which is why the "advise/advice" hint is very useful. As an amateur musician, I'm very familiar with the word in both forms.